LoRa (Long Range) was commercialised in 2014 by Semtech as a proprietary chirp spread spectrum (CSS) modulation. LoRaWAN, built on top of it, is the MAC-layer standard maintained by the LoRa Alliance. CSS is robust against Doppler shift and narrow-band interference and operates well at low SNR.

Chirp Spread Spectrum modulation

CSS uses signals whose frequency rises or falls linearly across the channel (chirps). Because symbol energy is spread over a wide frequency band, it can be decoded near or even below the noise floor. In LoRa the resolution is set by the spreading factor (SF): from SF7 (fastest, shortest range) to SF12 (slowest, most sensitive). Bandwidth is typically 125, 250 or 500 kHz.

Each step of SF doubles the symbol duration and adds, in theory, about 2.5 dB of sensitivity. At SF12 / 125 kHz the receiver sensitivity can drop to roughly −137 dBm. The price is throughput: data rate ranges from 0.3 kbps (SF12) up to 50 kbps (SF7).

Regional parameters and ISM bands

LoRa runs in unlicensed ISM (Industrial, Scientific, Medical) bands. Regional parameters are defined in the LoRa Alliance RP002 (Regional Parameters) document:

  • EU868 (Europe): 863–870 MHz, ETSI EN 300 220, with a 0.1–1% duty-cycle limit per sub-band.
  • US915 (North America): 902–928 MHz, FCC Part 15; instead of duty cycle, uses a dwell-time limit with frequency hopping.
  • AS923 (Asia): 920–925 MHz, dwell time 400 ms.
  • IN865 (India), AU915, KR920, RU864 and other regions use different channel plans.

In Europe a 1% duty cycle means a single channel may be occupied for at most 36 seconds per hour; this constraint discourages frequent transmission of large payloads at SF12.

Link budget and the Friis equation

The basis of any wireless range calculation is the link budget: transmitter output + antenna gains − path loss − fading margin ≥ receiver sensitivity. A typical LoRa link budget is in the 150–157 dB range, well above consumer Wi-Fi (~100 dB) and cellular systems (~140–145 dB).

Free-Space Path Loss (FSPL) is a consequence of the Friis equation:

FSPL (dB) = 20·log10(d) + 20·log10(f) + 32.44  (d in km, f in MHz)

At 868 MHz, FSPL is about 91 dB at 1 km and about 111 dB at 10 km. These are theoretical line-of-sight (LOS) values only; in the real world, models such as ITU-R P.1411 (the urban-area propagation model) are used.

Fresnel zone and obstacles

The radius of the first Fresnel zone between two antennas is:

r = 8.66 · √(d / f)  (r in m, d in km, f in GHz, at the midpoint)

At 868 MHz over 5 km, the midpoint Fresnel radius is roughly 16 m. More than 60% of this zone is expected to be clear; otherwise, diffraction losses quickly consume the link budget. Concrete, glass façades and metal roofing absorb or reflect the signal; multipath reflections introduce phase distortion.

Real-world range: documented LoRa records under open, unobstructed LOS sit at 10–15 km, with balloon and flight experiments reaching 700+ km. In a typical urban environment the range falls to 1–3 km, and to 300–800 m in dense city centres. A common rule of thumb is to divide the open-field range by 4–5 to estimate the urban expectation.

LoRaWAN topology and classes

LoRaWAN uses a star-of-stars topology; devices connect directly to gateways and there is no mesh. Three device classes are defined:

  • Class A: after each uplink the device opens two short downlink windows. Lowest power consumption; designed for sensors that must run 5–10 years on a battery.
  • Class B: scheduled receive windows are opened on the back of periodic gateway beacons; medium latency.
  • Class C: the device listens continuously except while transmitting; low latency, high power draw.

Alternative LPWAN technologies

Other low-power wide-area networks competing in the same market: Sigfox (ultra-narrow band, 100 bps, 12-byte payload), NB-IoT (3GPP Release 13, licensed spectrum, ~20 dB MCL gain), LTE-M (LTE Cat-M1) (with voice support) and the mesh-based Wi-SUN (IEEE 802.15.4g). Selection is driven by coverage, latency, power budget and the operator/licensing model.